The present invention generally relates to electrochemical battery cells having an oxygen reduction electrode. More particularly, the present invention relates to air-depolarized and air-assisted cells which use oxygen contained in air from outside the cells.
Air-depolarized cells are cells that use oxygen from the surrounding atmosphere to produce electrochemical energy. Oxygen diffuses into the cell and is used as the active material, or reactant, of the positive electrode (cathode). The cathode, also referred to as the air electrode, contains a catalyst that promotes reaction of the oxygen with the electrolyte and ultimately the negative electrode (anode) active material. Because the cathode active material comes from outside the cell, the air electrode is not consumed during discharge of the cell. This means that the volume of the air electrode can be small compared to a cell in which the cathode active material is limited to that which is put into the cell when it is manufactured. Consequently, in an air-depolarized cell, referred to hereafter as an air cell, a larger proportion of the total cell volume can be devoted to the anode, and the volumetric energy density of the cell is increased. This is a desirable feature for batteries used to power electronic devices which have limited space for the batteries. Air depolarized cells can be made in a variety of sizes and shapes, from small button cells to large cylindrical cells for example. Primary aqueous alkaline zinc/air batteries are commercially available in both button and larger sizes. Rechargeable zinc/air batteries are also known. Other metals, such as aluminum, magnesium, iron, lithium and calcium, can also be used as active anode materials in air cells. Types and characteristics of air cells are disclosed in chapters 13 and 38 of Handbook of Batteries, Second Edition, David Linden (ed.), McGraw-Hill, Inc., New York, 1995. Examples of button alkaline zinc/air cells are disclosed in U.S. Pat. No. 6,602,629 (issued Aug. 5, 2003, to Guo et al.); U.S. Pat. No. 6,551,742 (issued Apr. 22, 2003, to Huq et al.) and U.S. Pat. No. 5,721,065 (issued Feb. 24, 1998, to Collien et al.) as well as U.S. Patent Publication No. 2002/0192545 (by Ramaswami et al., published Dec. 19, 2002), all of which are hereby incorporated by reference. Examples of larger cylindrical alkaline zinc/air cells are disclosed in U.S. Pat. No. 6,461,761 (issued Oct. 8, 2002, to Moy et al. and U.S. Pat. No. 5,518,834 (issued May 21, 1996, to Yoshizawa et al.), and in U.S. Patent Publication No. 2002/0160251 (by Chang et al., published Oct. 31, 2002), all of which are hereby incorporated by reference.
Air-assisted cells are hybrid cells that contain cathodes that are consumed during discharge. Air-assisted cells can have an air electrode that also contains a significant amount of an active material, such as manganese dioxide, as disclosed in Handbook of Batteries, Second Edition, pages 38.10-38.12. At high discharge rates that cannot be sustained by the air electrode, the manganese dioxide functions as the active cathode material, and at low rates oxygen is the primary cathode active material. The manganese dioxide is partially regenerated by air oxidation at low rates and during periods of rest. Alternatively, air-assisted cells can have a cathode containing a consumable active material that is reoxidized, or recharged, by a separate air electrode when the cell is not being discharged or is being discharged at a sufficiently low rate. Examples of air-assisted alkaline zinc cells are disclosed in U.S. Pat. No. 6,383,674 (issued May 7, 2002, to Urry); U.S. Pat. No. 5,270,128 (issued Dec. 14, 1993, to Reichert et al.); U.S. Pat. No. 5,229,223 (issued Jul. 10, 1993, to Hyland) and U.S. Pat. No. 5,079,106 (issued Jan. 7, 1992 to Urry), all of which are hereby incorporated by reference.
The housings of air and air-assisted cells are generally not hermetically sealed, in order to provide a passageway for air from outside the cell to enter the cell so it can reach the oxygen reduction electrode. One or more air inlets can be provided for this purpose. An air inlet can be in the form of an aperture, a high permeability member, a tube or other type of passageway for air to enter the cell through the cell housing. A disadvantage of air and air-assisted cells is that electrolyte can leak through the air inlets due to, for example, manufacturing defects, failure of an internal seal and excessive internal pressure. The potential for leakage becomes greater when the potential for gas generation within the cell is greater, such as in alkaline cells with a zinc anode active material and little or no added mercury.
Previous attempts have been made to reduce or eliminate electrolyte leakage through the air inlets, but none of these has proven completely satisfactory. General approaches that have been used include: forming a more effective seal at the periphery of the air electrode, using more hydrophobic materials for the hydrophobic layer on the “air” (air entry) side of the air electrode, using purer electrode materials, plating anode current collectors or using additives that will reduce the rate or amount of gas generated inside the cell, better containing within the cell electrolyte that gets to the air side of the air electrode and providing a larger void space within the anode-containing portion of the cell to better accommodate increasing material volume, condensed water vapor from outside the cell and gas generated within the cell.
In U.S. Reissue Pat. No. Re. 31,413 (issued Oct. 11, 1983), Jaggard discloses a button type gas depolarized cell in which the insulator (gasket) is held in pressure contact against the peripheral portion of the cathode assembly to form an electrolyte seal to keep electrolyte from moving around the peripheral edge of the air electrode and into the area between the hydrophobic member and the inside surface of the can bottom. Jaggard also discloses the use of a blotter placed on the gas access side of the hydrophobic layer to act as an absorber for any electrolyte which may leak from the cell under extreme environmental conditions.
In U.S. Pat. No. 6,558,828 (issued May 6, 2003), Guo discloses an button alkaline zinc-air cell having a hydrophobic layer porosity controlled during lamination to the air electrode mixture to reduce water transmission both into and out of the cell, thereby improving cell performance in high and low humidity environments. Reducing the amount of water that enters the cell also reduces the buildup of internal pressure that can lead to cell bulging and leakage.
In U.S. Pat. No. 5,279,905 (issued Jan. 18, 1994) and U.S. Pat. No. 5,306,580 (issued Apr. 26, 1994), Mansfield Jr. et al. disclose an alkaline zinc/air button cell with little or no added mercury. The anode cup is made from a triclad material with a layer of copper on the inside surface and indium electroplated over the copper to reduce hydrogen gassing.
In U.S. Pat. No. 6,602,629 (issued Aug. 5, 2003), Guo et al. disclose an alkaline zinc/air button cell with no added mercury that is resistant to leakage and salting. A low-gassing zinc composition, containing a zinc-lead alloy with low levels of contaminants, is used, together with an inorganic gassing inhibitor (In(OH)3) added to the anode and zinc oxide and an organic surfactant (perfluoroalkyl polyethylene oxide) added to the electrolyte to reduce hydrogen gassing. The cell also has a copper-clad anode cup with no non-in situ-deposited metal with a hydrogen overvoltage higher than that of copper in the seal area to provide a smooth surface to minimize electrolyte capillary action through the gasket-anode cup interface to the outside of the cell.
U.S. Pat. No. 4,500,614 (issued Feb. 19, 1985, to Nagamine et al.) discloses the use of zinc alloyed with at least two of gallium, indium and thallium to minimize gassing in order to reduce the amount of mercury.
In U.S. Pat. No. 4,369,568 (issued Jan. 25, 1983), Dopp discloses the use of a void space between the anode cup and the anode material that is sufficient to accommodate all of the expanded anode material.
Unexamined Japanese Patent Publication No. 06-349,529 A (published Dec. 22, 1994), discloses a zinc/air button cell with acrylic fiber material between the air electrode and air access holes in the can to absorb electrolyte entering the area between the air electrode and the can and prevent leakage through the air holes.
Chinese Patent Publication No. 1,366,356 A (published Aug. 28, 2002), discloses a zinc-air battery with a water-absorbing resin layer within the container to absorb electrolyte so it will not leak outside the container.
Previous attempts to reduce or eliminate electrolyte leakage through the air inlets of air-depolarized and air-assisted cells have suffered from one or more problems. They may not eliminate the possibility of electrolyte passing around or through the air electrode, may not prevent excessive pressure from building up inside the cell, may significantly limit the high rate discharge capability of the cell, may reduce the amount of active material in a cell with limited volume and may not completely contain electrolyte that gets past the air electrode. Similar problems can exist in button type alkaline zinc/air cells as well as other types, sizes and shapes of air-depolarized and air-assisted cells.
In view of the above, an object of the present invention is to provide a cell with an oxygen reduction electrode having improved resistance to electrolyte leakage, as well as a high discharge capacity and good high rate discharge characteristics. A further object of the invention is to provide a cell with an oxygen reduction electrode that has improved electrolyte leakage characteristics and can be easily manufactured on high-volume, high-speed equipment at a reasonable cost.